Fuel cell, method for operating full cell and fuel cell system

ABSTRACT

A fuel cell, a method for operating a fuel cell and a fuel cell system, which ensure no dew condensation for a wet reaction gas in the inlet area of gas channels in plates in a fuel cell stack, are provided. Gas channels  2  and heat medium channels are disposed on one surface and the other surface of one plate  1 , respectively. Gas channels are disposed on the other plate such that they face the gas channels  2  in the plate  1 . A gas inlet header  3  is disposed at the upper part of the gas channel  2  in the plate  1  and a heat medium inlet header is disposed at the upper part of the heat medium channels such that they face the gas inlet header on the other side. Cooling water such as heat medium is supplied from the heat medium supply manifold hole  7  to the heat medium inlet header, thereby warming up the same. The water vapor in the reaction gas (wet fuel gas) is prevented from being condensed in the inlet area of the gas channels  2  by heating up the gas inlet header by the heat conduction.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a fuel cell, which is capable ofpreventing the dew condensation at a reaction gas header in a plate byhumidifying the stacked cells in a fuel cell stack, wherein the heatdischarged from the fuel cell stack is efficiently used for humidifyingthe cells. The present invention also relates to a method for operatingsuch a fuel cell and to a fuel cell system, which are suitable for theoperation of such a fuel cell.

[0003] 2. Description of the Related Art

[0004] In a conventional polymer electrolyte fuel cell, an anode (fuelelectrode) and cathode (air electrode) are deposited respectively on onesurface and the other surface of a solid polymer electrolyte membrane toform a unified element as a cell (membrane electrode assembly), and aunit fuel cell is formed by clamping the cell between both a plate (aseparator) having concave groove-shaped fuel gas channels on the surfacefacing the anode and another plate (another separator) having concavegroove-shaped oxidant gas channels on the surface facing the cathode.Such a plurality of unit fuel cells are stacked and unified into asingle unit by fastening the unit fuel cells together, using a boltpassing through the unit fuel cells and end plates adapted onto bothends thereof. Thus, a fuel cell stack is formed by the unit cells. Inthe operation, a fuel gas (hydrogen gas or reforming gas composed ofmainly hydrogen) is supplied into the fuel gas channels and an oxidantgas (normally air) is supplied into the oxidant gas channels, so that aDC electric power is obtained from the electrochemical reaction whichtakes place via the solid polymer electrolyte membrane.

[0005] In such a polymer electrolyte fuel cell, it is required tohumidify the solid polymer electrolyte membrane in order to obtainproper proton conductivity during a period for generating the electricpower. In the prior art, therefore, the reaction gas (fuel gas and/oroxidant gas) is supplied into gas channels in the plate, afterhumidifying the reaction gas with a humidifier, so that the solidpolymer electrolyte membrane is maintained in a moist state. Inparticular, it is preferable that the solid polymer electrolyte membraneis humidified with the reaction gas at the dew point equal to thetemperature of the membrane or the cell temperature or higher in orderto obtain sufficiently high proton conductivity.

[0006] Regarding the method for supplying a reaction gas having a dewpoint near the cell temperature, U.S. Pat. No. 5,382,478 discloses amethod of humidifying the reaction gas, using a heat resulting from afuel cell in the state where cooling water for the fuel cell comes intocontact with the reaction gas via a water permeable membrane. Since,however, the evaporation heat significantly increases with the increaseof the temperature, the fuel cell is mostly operated at a temperature of65° C. to 70° C. A further increase of the temperature in the fuel cellrequires a greater difference between the dew point of the reaction gasand the temperature of the fuel cell.

[0007] However, for example, when a reaction gas having a dew point nearthe cell temperature is supplied to in such a plate A as shown in FIG.5, water vapor is condensed in a manifold B, and therefore the condensedwater, i.e., the dew clogs the inlet of the gas channel C, therebycausing the flow of the reaction gas to be interrupted. Even when thedew point of the reaction gas is set at a temperature smaller than thecell temperature to some extent in order to avoid the above phenomenon,the dew condensation still takes place in gas channels C in response tothe consumption of the reaction gas, so that the dew clogs the gaschannels C and the supply of the reaction gas is suppressed. As aresult, the reaction gas is not uniformly distributed, and therefore theamount of the reaction gas to be supplied to the electrode becomesinsufficient and further the generation of the electric power is notnormally carried out, thereby causing the performance of the fuel cellto be deteriorated. In particular, the gas flow resistance becomeslarger in the vicinity of curved sections in the gas channels C, so thatthe condensed water is adhered thereto, thereby causing the gas channelsto be clogged. In order to avoid this fact, Japanese Patent PublicationNo. 2761059 discloses a technical measure, in which, for example,S-shaped gas channels are replaced with those in the form of straightline and the condensed water is moved to downstream by supplying thereaction gas from top to bottom in the direction of gravity, and inwhich each water supplying channel is further interposed between theadjacent gas channels to enhance an efficiency in cooling the fuel cellstack. Moreover, as for means for preventing the deterioration of thepower generation performance resulting form the condensed water,Japanese Unexamined Patent Application Publication No. 6-89730 disclosesa technical measure, in which, for example, water absorbing elements aredisposed in the gas channels and/or a dry gas is supplied at the middleportion in the gas channels to remove the condensed water.

[0008] In the above-described prior arts, the moisture content in thesolid polymer electrolyte membrane and the temperature at the fuel cellstack can be maintained within predetermined ranges, so that anexcellent responsibility of transferring to a heavy load for the fuelcell stack can be obtained and therefore a high output can be obtainedin a short time, thereby enabling a stable operation to be ensured forsuch a load variation. However, regarding the suppression of dewcondensation in the vicinity of the gas channel inlet in the case whenthe dew point of the reaction gas is increased up to a temperature nearthe cell temperature, satisfactory results cannot be always obtained.Regarding the countermeasure for the clogging of the reaction gaschannels due to the water condensed in the gas channels, theabove-described prior arts require either the insertion ofwater-absorbing material into the gas channels of the plate or themounting of holes and channels for supplying dry gas in the middleportion of the gas channels. This causes to provide a complicatestructure in the fuel cell and to require a lot of work for mountingthese components and for manufacturing the fuel cell. It may be stated,therefore, that no sufficient countermeasure is yet introduced into theprior arts.

[0009] Regarding a fuel cell system in the prior art, it is necessary todispose a heat source inside the fuel cell system in order to increasethe dew point of the reaction gas near the cell temperature. Moreover,when the dew point of the reaction gas is higher than the temperature ofthe fuel cell stack, the dew condensation takes place without delayafter the reaction gas is supplied to the fuel cell stack. Accordingly,it is necessary to control the relationship between the dew point of thereaction gas and the temperature of the fuel cell stack. The prior artmethod for humidifying the reaction gas, using the heat from the fuelcell, and bringing the cooling water for the fuel cell comes intocontact with the reaction gas via a water permeable membrane, ensuresthat the dew point of the reaction gas is always smaller than thetemperature of the fuel cell stack to some extent. In this case,however, the heat of evaporation strongly increases with the increase ofthe temperature, so that the fuel cell is operated at a temperature of65° C. to 70° C. In addition, almost all the heat of the cooling wateris used for humidification, so that it is difficult to recover the heatfrom the cooling water in the case of a cogeneration application.

SUMMARY OF THE INVENTION

[0010] Accordingly, it is an object of the present invention to providea polymer electrolyte fuel cell, which is equipped with plates having asimple structure, and, which is capable of being stably operated in thestate where the dew point of the reaction gas is increased close to thecell temperature.

[0011] It is another object of the present invention to provide a methodfor operating a polymer electrolyte fuel cell the same, which method issuitable for operating the same.

[0012] It is another object of the present invention to provide a fuelcell system, which includes a heat source for increasing the dew pointof the reaction gas such that it approaches the cell temperature, andwhich ensures an easy control of temperature and a high efficiency inrecovering the heat from the cooling water in the fuel cell.

[0013] Moreover, it is another object to provide a fuel cell system,which includes polymer electrolyte fuel cells capable of being operatedin the state where the reaction gas has a dew point having an increasedamount near the cell temperature.

[0014] To attain the above-mentioned objects, the following measures areprovided in the present invention.

[0015] In a first aspect of the invention, a fuel cell is formed bystacking a plurality of plates each having reaction gas channels or heatmedium channels in a fuel cell stack, wherein an inlet header for atleast one side of reaction gas channels is disposed so as to face theinlet header or the outlet header for said heat medium channels. Inaccordance with the first aspect of the invention, the temperature atthe reaction gas inlet header can be maintained by using the heatmedium. As a result, water vapor contained in the reaction gas is notcondensed and therefore no dew condensation is generated in the inletarea for the gas channels, thereby enabling a normal operation to beensured for the generation of electric power along with an enhancedperformance of the cell.

[0016] In a second aspect of the invention, it is included in the firstaspect of the invention that the inlet header for said reaction gaschannels is maintained at a temperature of the dew point of gas orgreater with aid of the heat medium. In accordance with the secondaspect of the invention, the dew condensation can be securely preventedat the reaction gas inlet header.

[0017] In a third aspect of the invention, a fuel cell is formed bystacking a plurality of plates each having reaction gas channels in afuel cell stack, wherein the inlet header for one side of reaction gaschannels is disposed such that it faces the inlet header for the otherside of reaction gas channels. In accordance with the third aspect ofthe invention, the one reaction gas inlet header can be maintained at apredetermined temperature by the other side of reaction gas.

[0018] In a fourth aspect of the invention, it is included in the thirdaspect of the invention that the dew point of the one side of reactiongas is set at the temperature of the other side of reaction gas orsmaller in the fuel cell according to the third aspect. In accordancewith the fourth aspect, the inlet header for the one side of reactiongas can be maintained at a temperature of the dew point or greater bythe other side of reaction gas to prevent dew condensation.

[0019] In a fifth aspect of the invention, a fuel cell is formed bystacking a plurality of plates each having reaction gas channels or heatmedium channels, wherein an inlet header for one side of reaction gasand an inlet header for the other side of reaction gas are disposed suchthat they face an inlet header or an outlet header for the heat medium.In accordance with the fifth aspect, the reaction gas inlet header canbe maintained at a predetermined temperature by the heat medium.

[0020] In a sixth aspect of the invention, it is included in the fifthaspect of the invention that the one side of reaction gas and the otherside of reaction gas flow parallel to each other from top to bottom inthe direction of gravity, and wherein both sides of reaction gas flow indirection parallel (co-flow) or anti-parallel (counter-flow) to the heatmedium. In accordance with the sixth aspect, the reaction gas can beefficiently heated up by the heat medium.

[0021] In a seventh aspect of the invention, it is included in the fifthor sixth aspect of the invention that channels for the one side of gas,the other side of gas and the heat medium are shaped straight in therespective portions facing an anode or cathode electrode section. Inaccordance with the seven aspect of the invention, the straight channelshaving no curved portion ensure a smooth flow of the reaction gas, alongwith a smooth discharge of the condensed water.

[0022] In an eighth aspect of the invention, it is included in one ofthe fifth to seventh aspects of the invention that the dew point of atleast one of the supplied reaction gases≦the temperature of heat mediumat the inlet in the case when at least one of the reaction gases and theheat medium flow in the direction parallel to each other, whereas thedew point of at lease one of the supplied reaction gases≦the temperatureof the heat medium at the outlet in the case when at least one of thereaction gases and the heat medium flow in the direction anti-parallelto each other. In accordance with the eight aspect of the invention, thedew condensation can be prevented at the reaction gas inlet header.

[0023] In a ninth aspect of the invention, it is included in the eightaspect of the invention that wherein a following equation is establishedin the case when at least one of the reaction gases and the heat mediumflow in the direction parallel to each other,

[0024] the dew point for at least one side of discharged reactiongas≧the temperature of the heat medium at the outlet,

[0025] whereas another equation is established in the case when at leastone of the reaction gases and the heat medium flow in the directionanti-parallel to each other,

[0026] the dew point for at least one side of discharged reactiongas≧the temperature of the heat medium at the inlet. In accordance withthe ninth aspect of the invention, the electrode portions can behumidified by condensing the water in the gas channels. Moreover, thevariation in the pressure loss can be reduced by dew condensation in allthe gas channels, thereby enabling the gas distribution to behomogenized. For instance, an increased gas flow rate in part of the gaschannels causes the amount of condensed water to be increased. Thiscauses the pressure loss to be increased and further the gas flow rateto be lowered.

[0027] In a tenth aspect of the invention, it is included in the thirdaspect of the invention that the heat medium is supplied so as to flowat an area facing the downstream area for the reaction gas inlet header,wherein the heat medium heat-exchanged at an area facing the electrodesection is supplied so as to flow at an area facing the reaction gasinlet header. In accordance with the tenth aspect of the invention, thecell can be maintained in a humidified state by dew condensation in thereaction gas at the area facing the downstream area for the inlet headerfor the reaction gas, and at the same time, the dew condensation can beprevented by heating up the reaction gas at the inlet header for thereaction gas.

[0028] In an eleventh aspect of the invention, it is included in thetenth aspect of the invention that a following equation is established,the dew point for at least one side of reaction gas≧the temperature ofthe heat medium at the inlet. In accordance with the eleventh aspect ofthe invention, the effect resulting from the tenth aspect can beensured.

[0029] In a twelfth aspect of the invention, it is included in one ofthe first to eleventh aspects of the invention that a flow resistancegeneration section is disposed at the inlet for at least one side ofreaction gas. In accordance with the twelfth aspect of the invention,the flow of the reaction gas can be regulated, and the distribution ofreaction gas in the respective gas channels can be homogenized, so thatthe water dew condensed in the lower level area can be discharged to theoutlet of the gas channels by the pushing force.

[0030] In a thirteenth aspect of the invention, it is included in thetwelfth aspect of the invention that the reaction gas inlet headerincludes the flow resistance generation section. In accordance with thethirteenth aspect of the invention, the effect resulting from thetwelfth aspect can be ensured. Since, moreover, the flow resistancegeneration section can be maintained at a temperature equal to orgreater than the dew point of the reaction gas, the dew condensation canbe prevented there.

[0031] In a fourteenth aspect of the invention, it is included in anyone of the first to thirteenth aspects of the invention that an oxidanthumidifier and a fuel humidifier are connected to the fuel cellaccording to any one of the first to thirteenth aspects, and the heatmedium discharged from the fuel cell is heat-exchanged in thesehumidifiers. In accordance with the fourteenth aspect of the invention,a dew point of the reaction gas near the temperature of the cell can beobtained only by the heat in the heat medium in the case of a lowoperation temperature (for example, 70° C. or less).

[0032] In a fifteenth aspect of the invention, an oxidant humidifierand/or a fuel humidifier, and a total heat exchanger are connected to afuel cell, and the heat medium discharged from the fuel cell isheat-exchanged in these humidifiers, wherein the total heat exchange iscarried out between at least one side of reaction gas in the reactiongas discharged from the fuel cell and at least one side of reaction gasin the reaction gas before supplied to said humidifiers. In accordancewith the fifteenth aspect of the invention, a humidifying temperaturenear the temperature of the cell can be obtained by the heat in the heatmedium and by using the heat recovered from the gas discharged from thefuel cell, in the case of a high cell temperature (for example, 70° C.or more).

[0033] In a sixteenth aspect of the invention, the fuel cell and themethod of operating the same employed in the fuel cell system accordingto the fifteenth aspect of the invention are characterized respectivelyby a fuel cell and a method for operating the same according to any oneof the first to fifteenth aspects of the invention. In accordance withthe sixteenth aspect, the effect resulting from the first to fifteenthaspects can be ensured.

[0034] In a seventeenth aspect of the invention, it is included in anyone of the fourteenth to sixteenth aspects of the invention that theheat medium discharged from the fuel cell is first heat-exchanged in onehumidifier in which either the oxidant gas or fuel gas flows at a higherflow rate, and then heat-exchanged in the other humidifier. Inaccordance with the seventeenth aspect of the invention, when the air isused as an oxidant gas, the heat exchange is firstly carried out in theair humidifier due to a greater flow rate of air. Accordingly, thedifference between the dew point of the oxidant gas and that of the fuelgas can be reduced, thereby enabling the performance of the cell to beenhanced.

[0035] As described above, the present invention is us fully applicableto both a polymer electrolyte fuel cell and a method for operating thesame. The present invention is also applicable to a fuel cell system,which is used in a power generation system with such a polymerelectrolyte fuel cell, a cogeneration system and others.

[0036] Further objects, features and advantages of the present inventionwill become apparent form the following description of the preferredembodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a schematic plan view of a fuel cell in a firstembodiment of the invention, where reaction gas and cooling waterflowing in channels of a plate are represented in a perspective view;

[0038]FIG. 2 is a schematic plan view of a fuel cell in a secondembodiment of the invention. where reaction gas and cooling waterflowing in channels of a plate are represented in a perspective view;

[0039]FIG. 3 is a schematic plan view of a fuel cell in a thirdembodiment of the invention, where reaction gas and cooling waterflowing in channels of a plate are represented in a perspective view;

[0040]FIG. 4(a) is a plan view of an embodiment of a flow resistancegeneration section mounted in a fuel cell according to the invention;

[0041]FIG. 4(b) is a front view of the same;

[0042]FIG. 5 is a plan view of a plate used in a conventional fuel cell;

[0043]FIG. 6 is a schematic sectional view of a component in a fuel cellstack used in a fuel cell system according to the invention;

[0044]FIG. 7(a) is a plan view of a bipolar plate mounted in a fuel cellstack according to the invention viewed from the side of the fuel gaschannels;

[0045]FIG. 7(b) is a plan view of the same viewed from the oxidant gaschannel side;

[0046]FIG. 8(a) is a plan view of an anode cooling plate mounted in afuel cell stack according to invention viewed from the side of the fuelgas channels;

[0047]FIG. 8(b) is a plan view of the same viewed from the side of thewater channels;

[0048]FIG. 9(a) is a plan view of a cathode cooling plate mounted in afuel cell stack according to the invention viewed from the side of theoxidant gas channels;

[0049]FIG. 9(b) is a plan view of the same an the side on which no gaschannels are formed; and

[0050]FIG. 10 is a block diagram of a fuel cell system in an embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] Referring now to the accompanying drawings, several embodimentsof a fuel cell according to the invention will be described.

[0052] (Embodiment 1)

[0053]FIG. 1 is a plan view of a fuel cell according to the invention ina first embodiment, where the reaction gas and cooling water flowing incorresponding channels arranged in a plate are schematically representedin a perspective view. In FIG. 1, reference numeral 1 means the platetypically made of carbon, where a plurality of concave groove-shaped gaschannels 2 is formed on one surface of the plate so as to align in theup/down direction (the direction of gravity) and a plurality of concavegroove-shaped water channels is formed on the other surface of the plateso as align in the up/down direction (the direction of gravity) so as toface each other. In this case, a gas supply manifold hole 3 is disposedon the upper left side of the plate 1 so as to pass therethrough, andthe gas supply manifold hole 3 is connected to a gas inlet header 4 inthe concave form. Moreover, the gas inlet header 4 is further connectedto the gas channels 2. The gas inlet header 4 is defined as an areawhere a reaction gas (fuel gas or oxidant gas) supplied in a distributedstate from the gas supply manifold hole 3 is further supplied to theinlet of the gas channels 2 (the same definition is applied in thefollowing). Such a gas inlet header 4 is generally called as a manifold.

[0054] The outlet of the gas channels 2 is connected to a gas outletheader 5 in the concave form, which is disposed in the lower part of theplate 1, and the gas outlet header 5 is connected to a gas dischargemanifold hole 6 which is disposed on the lower left side of the plate 1so as to pass therethrough. As a result, a reaction gas (in this case,fuel gas) is supplied and distributed to the gas inlet header 4 of theplate 1 in each cell via the gas supply manifold hole 3, which isaligned in the stacking direction of the fuel cell stacks, and thendistributed from the gas inlet header 4 to the gas channels 2, so thatthe reaction gas is supplied from top to bottom in the gas channels 2,and then discharged from the gas outlet header 5. At the same time, thesupplied reaction gas flows into a gas discharge manifold hole 6 alignedin the stacking direction of the fuel cell stack, and finally dischargedto the outside from the end portion of the fuel cell stack via the gasdischarge manifold hole 6.

[0055] On the upper right side of the plate 1 (the side opposite to thegas supply manifold hole 3), a water supply manifold hole 7 is disposedso as to pass through the plate, and the water supply manifold hole 7 isconnected to a water inlet header in the concave form, which is disposedon the other surface of the plate 1. The water inlet header is furtherconnected to the inlet of the water channels. In this case, the gasinlet header 4 and the water inlet header are disposed respectively onthe one surface and the other surface of the plate 1 so as to face eachother.

[0056] Moreover, a water outlet header in the concave form is disposedat the outlet of the water channel in the plate 1, and the water outletheader is connected to a water discharge manifold hole 8, which isdisposed on the lower right side of the plate 1 (on the side opposite tothe gas discharge manifold hole 6) so as to pass therethrough. As aresult, the water supplied from the end portion of the fuel cell stack(in this case, cooling water) is supplied and distributed to the waterinlet header of the plate 1 in the respective cells through the watersupply manifold 7, which is aligned in the stacking direction of thefuel cell stack. The supplied water is distributed in water channelsfrom the water inlet header and is supplied from top to bottom along thewater channels, and then discharged into the water outlet header, andfurther flows into the water discharge manifold hole 8, which is alignedin the stacking direction of the fuel cell stack. Finally, the water isdischarged to the outside from the end portion of the fuel cell stackafter passing through the water discharge manifold hole 8.

[0057] On the other hand, a plurality of gas channels corresponding tothe gas channels 2 in the plate 1 is disposed from top to bottom (in thedirection of gravity) in the other plate. A gas inlet header 4′ in theconcave form is connected to the inlet of the gas channels and a gasoutlet header 5′ in the concave form is connected to the outlet of thegas channels. As a result, in the other plate, an oxidant gas (In thiscase, air introduced from the outside air) is supplied to the gas inletheader 4′, and distributed into the gas channels from the gas inletheader 4′. Then, the oxidant gas flows from top to bottom along the gaschannels, and discharged to the gas outlet header 5′, and finallydischarged to the outside of the fuel cell stack.

[0058] A cell is inserted between the gas channels 2 of the plat 1 andthe gas channels of the other plate, and the composite member thusformed is mounted in the fuel cell stack. In this case, a unit cell isconstituted by contacting closely and facing the cathode of the cell tothe gas channels 2 of the plate 1 and by contacting closely and facingthe cathode of the cell to the gas channels of the other plate. Then,the fuel cell stack is constituted by stacking the unit cells to form aunit unit. Regarding the gas inlet headers 4, 4′, the gas outlet header5, 5′, the water inlet header and the water outlet header, the uppersurface of the concave portion is covered by a gasket or the like,thereby enabling the leakage of gas and water to be prevented.

[0059] In the fuel cell thus constituted, the fuel gas flows in the gaschannels 2 of the plate 1 and the oxidant gas flows in the gas channelsof the other plate. As a result, an electrochemical reaction takes placevia the polymer electrolyte membrane of the cell, thereby enabling theDC electric power to be generated.

[0060] In order to humidify the polymer electrolytic membrane of thecell in the saturated state, the fuel gas is supplied to the fuel cellstack, after it is humidified with, for example, a humidifier at a dewpoint close to the cell temperature. In the prior art, a wet fuel gas iscooled particularly in the inlet area of the gas channels 2, when it issupplied to the gas channels 2, so that the water vapor contained in thegas is dew condensed to form the dew. As a result, the condensed wateris adhered to inside wall of the gas channels 2 and clogs them, therebycausing the flow of the fuel gas to be interrupted. In this embodiment,however, the water inlet header is disposed so as to be close to the gasinlet header 4 so as to face each other. As a result, the water inletheader is heated by the cooling water as a heat medium supplied thereto,and the gas inlet header 4 is indirectly heated up by the heatconduction, thereby making it possible to prevent the water vaporcontained in the fuel gas from dew condensation.

[0061] In order to suppress the dew condensation by the cooling water asa heat medium, the temperature of the cooling water should be set at thedew point of the fuel gas or greater (the dew point of the fuel gas≦thetemperature of the cooling water at the inlet). Furthermore, it ispreferable that the temperature of the oxidant gas (air) is set so as tofulfill the following relationship: The dew point of the fuel gas≦thedew point of the air≦the temperature of the cooling water at the inlet.

[0062] In this embodiment, the cooling water is used as a heat mediumfor preventing the dew condensation in the fuel gas. However, theoxidant gas can be used for a heat medium instead of the cooling water.In this case, the gas inlet header for the oxidant gas is disposed closeto the gas outlet header 4 for the fuel gas in the plate 1 on the otherside, although the arrangement is not shown, and water channels forsupplying the cooling water are disposed in the other plate.Furthermore, in order to prevent the dew condensation of the fuel gas bythe oxidant gas (air), the temperature of the air inlet is set such thatthe dew point of the fuel gas≦the temperature of the air.

[0063] (Embodiment 2)

[0064]FIG. 2 is a plan view of a fuel cell according to the invention ina second embodiment, where the reaction gas and cooling water flowing incorresponding channels arranged in a plate are schematically representedin a perspective view. This embodiment is different from the firstembodiment as for the point that an inner air manifold system isemployed in the second embodiment. In FIG. 2, reference numeral 1 meansa plate made of mainly carbon. A plurality of gas channels 2 in the formof concave grooves are disposed from top to bottom (in the direction ofgravity) on one surface of the plate 1 and a plurality of water channelsin the form of concave grooves are disposed from top to bottom (in thedirection of gravity) on the other surface of the plate 1 in such a waythat the gas channels 2 face the water channels. In this case, a gassupply manifold hole 3 is disposed on the upper left side of the plate 1in such a way that it passes through the plate 1, and the gas supplymanifold hole 3 is connected to a gas inlet header 4 in the concaveform. Moreover, the gas inlet header 4 is connected to the gas channels2. Such a gas inlet header 4 is generally called as a manifold.

[0065] The outlet of the gas channels 2 is connected to a gas outletheader 5 in the form of a concave shape, and the gas outlet header 5 isconnected to a gas discharging manifold hole 6, which is disposed on thelower left side of the plate 1 so as pass therethrough. As a result, areaction gas (fuel gas) supplied from the end portion of the fuel cellstack is distributed to the gas inlet header 4 of the plate 1 in eachcell via the gas supply manifold hole 3 which is aligned in the stackingdirection of the fuel cell stack, and the reaction gas is furtherdistributed from the gas inlet header 4 into the gas channels 2, so thatthe gas thus distributed flows from top to bottom along the gas channels2, and it is discharged to the gas outlet header 5 and further flowsinto the gas discharge manifold hole 6 aligned in the stacking directionof the fuel cell stack. Finally, the reaction gas is discharged from theend portion of the fuel cell stack to the outside via the gas dischargemanifold hole 6.

[0066] In addition, a water supply manifold hole 7 is disposed on theupper right side of the plate 1 (on the side opposite to the gas supplymanifold hole 3). The water supply manifold hole 7 is connected to thewater inlet header in the form of a concave shape, which is disposed onthe other surface of the plate 1, and the water inlet header 7 isconnected to the inlet of the water channels.

[0067] Moreover, a water outlet header in the concave form is disposedin the outlet of the channels on the other surface of the plate 1, andthe water outlet header is connected to a water discharge manifold hole8, which is disposed on the lower right side of the plate 1 (on the sideopposite to the gas discharge manifold hole 6) so as pass therethrough.As a result, water (cooling water), which is supplied from the endportion of the fuel cell stack, is supplied and distributed to the waterinlet header in the plate 1 of each cell via the water supply manifoldhole 7 aligned in the stacking direction of the fuel cell stack, and thewater is further distributed from the water inlet header to the waterchannels. Thereafter, the water flows from top to bottom along the waterchannels, and it is discharged to the water outlet header and then flowsinto the water discharge manifold hole 8 aligned in the stackingdirection of the fuel cell stack. Finally, the water is discharged tothe outside from the end portion of the fuel cell stack via the waterdischarge manifold hole 8.

[0068] On the other hand, a plurality of gas channels corresponding tothe gas channels 2 in the plate 1 are arranged from top to bottom (inthe direction of gravity) in the other plate. In this case, a gas supplymanifold hole 3′ is disposed on the upper right side of the other plateso as to pass therethrough, and the gas supply manifold hole 3′ isconnected to a gas inlet header in the concave form, and further the gasinlet header is connected to the gas channels.

[0069] The outlet of the gas channels in the other plate is connected tothe gas outlet header in the concave form, which is disposed in thelower part of the plate, and the gas outlet header is further connectedto a gas discharge manifold hole 6′, which is disposed in the lower endof the other plate so as pass therethrough. As a result, oxidant gas(air) supplied from the end portions of the fuel cell stack is suppliedand distributed to the gas inlet header on the other plate in each cellvia the gas supply manifold hole 3′ aligned in the stacking direction ofthe fuel cell stack, and then distributed to the gas channels from thegas inlet header. The oxidant gas thus distributed flows from top tobottom along the gas channels, and it is discharged to the gas outletheader. Thereafter, the oxidant gas flows into the gas dischargemanifold hole 6′ aligned in the stacking direction of the fuel cellstack and is discharged from the end portion of the fuel cell stack tothe outside via the gas discharge manifold hole 6′.

[0070] Similarly to the first embodiment, each cell is inserted betweenthe gas channels 2 in the plate 1 and the gas channels in the otherplate, and the composite elements obtained after the insertion aremounted in the fuel cell stack. In this case, an anode in the cell facesthe gas channels 2 in the plate 1 and contacts closely thereto, and acathode in the cell faces the gas channels in the other plate andcontacts closely thereto, so that a unit cell is formed. The fuel cellstack is produced by stacking such unit cells to form a unit. In thiscase, the gas inlet header 4, gas outlet header 5, water inlet headersand the water outlet header are covered on their concave upper surfaceby a gasket or the like, so that the leakage is prevented.

[0071] In the fuel cell stack thus formed in the second embodiment, thefuel gas flows into the gas channels of the plate 1 and the oxidant gasflows into the gas channels of the other plate, so that theelectrochemical reaction takes place via the polymer electrolytemembrane of the cell, thereby enabling a DC electric power to begenerated.

[0072] As described above, in order to humidify the polymer electrolytemembrane of the cell in the saturated state, a fuel gas is supplied tothe fuel cell stack after humidified with, for example, a humidifiersuch that the dew point is close to the cell temperature. In the secondembodiment, the water inlet header is disposed such that it is close tothe gas inlet header 4 on the other side, so that the water inlet headeris heated by the cooling water supplied thereto, and the gas inletheader 4 is indirectly heated by the heat conduction, thereby making itpossible to prevent the water vapor contained in the fuel gas fromcondensing. Accordingly, the clogging of the fuel gas due to thecondensed water can be suppressed, and a normal operation in thegeneration of the electric power is ensured, thereby enabling highperformance of the cell to be maintained.

[0073] In order to suppress the dew condensation resulting from thecooling water, the inlet for the cooling water is set at the dew pointof the fuel gas or grater (the dew point of the fuel gas≦the temperatureof the cooling water at the inlet). Furthermore, it is preferable thatthe temperature relationship for the oxidant gas (air) is set such thatthe dew point of the fuel gas≦the dew point of the air≦the temperatureof the cooling water at the inlet. If, moreover, the cooling water atthe outlet is set at a temperature equal to or smaller than the dewpoint of the reaction gas at the outlet, the solid polymer electrolytemembrane can be securely humidified. In this case, since the dewcondensation occurs in all the gas channels, the deviation in thepressure loss is reduced and therefore a uniform gas distribution can beobtained.

[0074] In the second embodiment, the cooling water is also used as aheat medium for preventing the dew condensation of the fuel gas.However, the oxidant gas can be used as a heat medium instead of thecooling water. In this case, the gas inlet header for the oxidant gas isdisposed close to the gas outlet header 4 for the fuel gas in the plate1 on the other side, although the arrangement is not shown, and waterchannels for supplying the cooling water are disposed in the otherplate. Furthermore, in order to prevent the dew condensation of the fuelgas by the oxidant gas (air), the temperature of the air inlet is setsuch that the dew point of the fuel gas≦the temperature of the air.

[0075] In the first and second embodiments, the reaction gas and thecooling water flow in the direction parallel to each other as well as inthe direction of gravity at the area facing the electrodes. However, itis possible to employ the structural arrangement in which the reactiongas and the cooling water flow in the direction anti-parallel to eachother. In this case, the relationship, the dew point of the fuel gas≦thedew point of the air≦the temperature of the cooling water at the outlet,is preferably set.

[0076] (Embodiment 3)

[0077]FIG. 3 is a plan view of a fuel cell according to the invention ina third embodiment, where the reaction gas and cooling water flowing incorresponding channels arranged in a plate are schematically representedin a perspective view. In FIG. 3, the structure of plate 1 is basicallysimilar to that in the second embodiment. As a result, the samereference numeral is attached to the same structural element as in thesecond embodiment and therefore detailed description thereof is omitted.Hence, detailed description is given exclusively to the structuralelements different from those in the second embodiment. A maindifference between the second and third embodiments is that a flowresistance generation section 9 is disposed at the inlet area of gaschannels 2 in a plate 1.

[0078] The flow resistance generation section 9 has, for example, such astructure as shown in FIG. 4. FIG. 4(a) is a plan view of the flowresistance generation section 9 and FIG. 4(b) is a front view of thesame. The flow resistance generation section 9 is formed by a thinplate-like base plate 9A having a connection section 9B, in whichprojection pieces 91 in the form of teeth are arranged with apredetermined spacing in one end of the base plate 9A. In this case, anozzle hole 92 is disposed in each projection piece 91 in such a waythat it passes through the center thereof from the other end of the baseplate 9A.

[0079] The flow resistance generation section 9 can be formed in aunified body from a material, which is selectable from synthetic resin,such as polyacetal, polymethylpentene, polyphenylene ether, polyphenylensulfide and liquid crystal polymer. Any resin for the material can beused, so long as it provides an excellent fluidity in the moldingprocess, a high precision in the finishing, an appropriate flexibilityand an excellent thermal conductivity.

[0080] The flow resistance generation section 9 is designed, as for thesize, to fit on the concave portion (not shown) in the inlet of the gaschannels 2, and as for the thickness, such that the upper surface of thesection 9 is located at the same level as the upper surface of the plate1, when it is fitted on the concave portion. The flow resistancegeneration section 9 is mounted onto the concave portion by adhesion. Inthis case, the mounting is carried out such that the projection pieces91 are inserted into the corresponding flow channels in the gas channels2. As a result, the gas inlet header 4 and the gas channels 2 aresecurely connected to each other via the nozzle holes 92. The diameterof the nozzle hole 92 is about 0.25 mm on the side of the inlet (on theside of the gas inlet header 4), and 0.22 mm on the side of the outlet(on the side of the gas channels 2)., and each nozzle hole 92 is taperedto some extent such that the gas passing through the hole can be ejectedtherefrom.

[0081] Aside from the water supply manifold hole 7, a second watersupply manifold hole 10 is disposed in the plate 1 so as to passtherethrough. Cooling water is supplied from the second water supplymanifold hole 10, and in the water channels on the other surface of theplate 1, the cooling water is introduced into an area located somewhatdownstream from the flow resistance generation section 9. A differencebetween the second and the third embodiments also resides in such astructural arrangement.

[0082] Moreover, a second water discharge manifold hole 11 is disposedon the upper left side of the plate 1 (on the side opposite to the watersupply manifold hole 7) so as to pass therethrough, and it is connectedto the water inlet header. In this case, the water inlet header isseparated from the inlet of the water channels for supplying the coolingwater by disposing a partition wall (not shown) in the interface to theinlet of the water channels. A difference between the second and thethird embodiments also resides in such a structural arrangement.

[0083] In the third embodiment, the cooling water is supplied from thesecond water supply manifold hole 10 to the water channels in the plate1, and the flows from top to bottom in the water channels. Thereafter,the cooling water is discharged from the outlet of the water channels tothe water supply manifold hole 8, and it is further supplied from thewater discharge manifold hole 8 to the water supply manifold hole 7.Moreover, the cooling water is supplied to the water inlet header anddischarged from the water inlet header to the second water dischargemanifold hole 11, and then flows in the stacking direction of the fuelcell stack, and finally discharged from the end portion of the fuel cellstack to the outside.

[0084] In the above water circulating channel, the means for supplyingthe cooling water from the water discharge manifold hole 8 to the watersupply manifold hole 7 can be realized, for example, by concavegroove-shaped channels (not shown) which are connected to the waterdischarge manifold hole 8 and to the water supply manifold hole 7 on theother surface of the plate 1, or by a tube-shaped connection channeldisposed either in the end plate of the fuel cell stack or outside thefuel cell stack such that the water discharge manifold hole 8 isconnected to the water supply manifold hole 7. In this case, the coolingwater is supplied in the water channels in the plate 1 and then returnedto the water supply header in the plate 1.

[0085] The reason why the cooling water is supplied from the secondwater supply manifold hole 10 is due to the fact that the polymerelectrolyte membrane in the cell connecting to the gas channel 2 ishumidified and maintained in the saturated moist state, in which case,the cooling water cools the inlet area for the water channels, andfurther cools the inlet area for the gas channels 2 facing the waterchannels on the other side, so that the dew point of the fuel gas islowered when the fuel gas is introduced into the gas channels 2, andthereby the water vapor contained in the fuel gas is compulsivelycondensed.

[0086] Furthermore, the reason why the cooling water passed through thewater channels in the plate 1 is again returned to the water supplyheader is due to the fact that the area surrounded by the broken line inFIG. 3 is warmed up, in which case, the flow resistance generationsection 9 is disposed in an area facing the above-mentioned area on theother surface, and the flow resistance generation section 9 is warmed upby the heat conduction, so that the dew condensation in the nozzle holes9 is prevented.

[0087] In the inlet area of the gas channels 2, the fuel gas ismaintained in the state where the dew condensation occurs easily,thereby making it possible to prevent the polymer electrolyte membranefrom being dried up. In this case, a fuel gas is ejected from the nozzlehole 92 of the flow resistance generation section 9, even if the watervapor is excessively condensed in the gas channel 2. Therefore, thecondensed water adhered to the inner wall of the gas channel 2 is blownoff, and it can be moved to the outlet at the downstream. As a result,the flow of the fuel gas is no longer hindered, because the condensedwater does not clog the gas channels 2, and therefor a reduction in theperformance of the fuel cell can be prevented before it happens.

[0088] In the third embodiment where the flow resistance generationsections 9 are equipped and the return of the cooling water is provided,it is preferable that the following conditions are set up: The dew pointof the air≧the dew point of the fuel gas≧the temperature at the inletfor the cooling water in the second water supply manifold hole 10; andat the same time, the dew point of the fuel gas≦the temperature at theinlet for the cooling water in the water supply manifold hole 7.

[0089] In the following, referring to the accompanying drawings, theembodiments of a fuel cell system according to the invention will bedescribed.

[0090]FIG. 6 is a schematic sectional view of a component in a fuel cellstack. In FIG. 6, reference numeral 21 means a bipolar plate. On oneside of the bipolar plate, a concave groove-shaped fuel gas channel 21 ais in parallel disposed in the form of a straight channel, and on theother side of the bipolar plate a concave groove-shaped oxidant gaschannel 21 b is also in parallel disposed in the form of a straightchannel.

[0091]FIG. 7(a) is a plan view of a bipolar plate 21 viewed from theside of the fuel gas channels. A concave gas inlet header 21 c connectedto each fuel gas channel 21 a is disposed in the inlet of the fuel gaschannel 21 a, and a fuel gas supply manifold 21 d is connected to thegas inlet header 21 c. Similarly, a concave gas outlet header 21 econnected to each fuel gas channels 21 a is disposed in the outlet ofthe fuel gas channel 21 a, and a fuel gas discharge manifold 21 f isconnected to the gas outlet header 21 e. Moreover, a nozzle-shaped flowresistance generation section 21 g is mounted in the inlet area of thefuel gas channels 21 a, so that the cross section of each fuel gaschannel 21 a is reduced. As a result, the fuel gas flows from the fuelgas supply manifold 21 d to the gas inlet header 21 c and then flowsinto each fuel gas channel 21 a after accelerated by the flow resistancegen ration section 21 g. Thereafter, the fuel gas is discharged from theoutlet of the fuel gas channels 21 a to the gas outlet header 21 ebefore coming together, and finally discharged into the fuel gasdischarge manifold 21 f.

[0092]FIG. 7(b) is a plan view of the bipolar plate 21 viewed from theside of the oxidant gas channels. A concave gas inlet header 21 hconnected to each oxidant gas channel 21 b is disposed in the inlet ofthe oxidant gas channels 21 b and a concave gas outlet header 21 iconnected to each oxidant gas channel 21 b is disposed in the outlet ofthe oxidant gas channels 21 b. In the inlet area of the oxidant gaschannels 21 b, a nozzle-shaped flow resistance generation section 21 jis mounted, so that the cross section of each oxidant gas channel 21 bis reduced. As a result, the oxidant gas of air flows into the gas inletheader 21 h, and then flows into each oxidant gas channel 21 b afteraccelerated by the flow resistance generation section 21 j. Thereafter,the oxidant gas is discharged from the outlet for the oxidant gaschannels 21 b to the gas outlet header 21 i, and then discharged to theoutside. In FIGS. 7(a) and 7(b), reference numerals 21 k and 21 m mean awater supply manifold and water discharge manifold, respectively.

[0093] In FIG. 6, reference numeral 22 means an anode cooling plate. Aconcave groove-shaped fuel gas channels 22 a are in parallel disposed ina straight groove on one surface of the anode cooling plate, andsimilarly a concave groove-shaped heat medium channels 22 b are inparallel disposed in a straight groove on the other surface of the anodecooling plate.

[0094]FIG. 8(a) is a plan view of the anode cooling plate 22 viewed fromthe side of the fuel gas channels. A concave gas inlet header 22 cconnected to each fuel gas channel 22 a is disposed in the inlet of thefuel gas channels 22 a, and the gas inlet header 22 c is connected to afuel gas supply manifold 22 d. Similarly, a concave gas outlet header 22connected to each fuel gas channel 22 a is disposed in the outlet of thefuel gas channels 22 a, and the gas outlet header 22 e is connected to agas discharge manifold 22 f. Moreover, a nozzle-shaped flow resistancegeneration section 22 g is mounted in the inlet area of the fuel gaschannels 22 a, so that the cross section of each fuel gas channel 22 ais reduced. As a result, the fuel gas flows from the fuel gas supplymanifold 22 d to the gas inlet header 22 c, and then flows into eachfuel gas channel after accelerated by the flow resistance generationsection 22 g. Thereafter, the fuel gas is discharged from the outlet ofthe gas channels 22 a to the gas outlet header 22 e before comingtogether, and then discharged to the fuel gas discharge manifold 22 f.

[0095]FIG. 8(b) is a plan view of the anode cooling plate 22 viewed fromthe side of the heat medium channels. A concave heat medium inlet header22 h connected to each heat medium channel 22 b is disposed in the inletfor the heat medium channels 22 b, and the heat medium inlet header 22 his connected to a heat medium supply manifold 22 k. Similarly, a concaveheat medium outlet header 22 i connected to each heat medium channel 22b is disposed in the outlet for the heat medium channels 22 b, and theheat medium outlet header 22 i is connected to a heat discharge manifold22 m. As a result, the heat medium of water flows from the heat mediumsupply manifold 22 k to the heat medium inlet header 22 h, and thenflows into each heat medium channel 22 b. Thereafter, the heat medium isdischarged from the outlet of the heat medium channels 22 b to the heatmedium outlet header 22 i before coming together, and finally dischargedto the heat medium discharge manifold 22 m.

[0096] The anode cooling plate 22 thus structured is arranged such thatthe surface of the anode cooling plate 22 on the side of the fuel gaschannels 22 a faces the surface on the side of the oxidant gas channels21 b of the bipolar plate 21, and that a cell (membrane electrodeassembly: MEA) is inserted between the above-mentioned surfaces. Then, agasket G is disposed in such a way that it surrounds the periphery ofthe cell.

[0097] In FIG. 6, reference numeral 23 means a cathode cooling plate. Aconcave groove-shaped oxidant gas channels 23 b are disposed in astraight and parallel flow channel on one surface of the cathode coolingplate 23.

[0098]FIG. 9(a) is a plan view of the cathode cooling plate 23 viewedfrom the side of the oxidant gas channels 23 b. A concave gas inletheader 23 h connected to each oxidant gas channel 23 b Is disposed inthe inlet for the oxidant gas channels 23 b, and a concave gas outletheader 23 i connected to each oxidant gas channel 23 b is disposed inthe outlet for the oxidant gas channels 23 b. A nozzle-shaped flowresistance generation section 23 g is mounted to the inlet area for theoxidant gas channels 23 b, and the cross section of each oxidant gaschannel 23 b is reduced. As a result, the oxidant gas of air flows intothe gas inlet header 23 h, and further flows into each oxidant gaschannel 23 b after accelerated by the flow resistance generation section23 g. Thereafter, the oxidant gas is discharged from the outlet for theoxidant gas channels 23 b to the gas outlet header 23 i and finallydischarged to the outside. FIG. 9(b) is a plan view of the cathodecooling plate 23, viewed from the side on which the oxidant gas channels23 b are not formed. In FIGS. 9(a) and 9(b), reference numerals 23 d, 23f, 23 k and 23 m mean a fuel gas supply manifold, fuel gas dischargemanifold, heat medium supply manifold and a heat medium dischargemanifold, respectively.

[0099] The cathode cooling plate 23 is positioned such that the side onwhich the oxidant gas channels 23 b are not formed faces the side of theheat medium channels 22 b in the anode cooling plate 22. Moreover,regarding the cathode cooling plate 23, the side on which the fuel gaschannels 21 a in the bipolar plate 21 having the same structure as inthe above-mentioned bipolar plate 21 are formed faces the side of theoxidant gas channels 23 b, and the bipolar plate 21 is positioned byinserting a cell (membrane electrode assembly: MEA) therebetween. Inthis case, a gasket G is also mounted such that it surrounds theperiphery of the cell.

[0100] The respective plates are combined with each other in theabove-mentioned sequence and then stacked. Moreover, end plates (notshown) are attached to both ends of the plates thus stacked, and thenfastened by rods or the like to form a fuel cell stack. In therespective plates, the fuel gas supply manifold, fuel gas dischargemanifold, heat medium supply manifold and the heat discharge manifoldprovide through holes aligned in the stacking direction of the fuel cellstack. Thus, the fuel cell stack can also be constituted exclusively bythe combination of an anode cooling plate and a cathode cooling platewithout any bipolar plate.

[0101]FIG. 10 is a block diagram of an embodiment of a fuel cell systemaccording to the invention. In this case, an air humidifier 25 isconnected to a heat medium discharge outlet 24 a of a fuel cell 24, anda fuel humidifier 26 is connected to the air humidifier 25. Furthermore,a heat exchanger 27 is connected to the fuel humidifier 26. Thus, awater circulation channel 28 for the cooling water as the heat medium isconstituted by connecting the heat exchanger 27 to a heat medium supplyopening 24 b in the fuel cell 24. Moreover, a total heat exchanger 29 isconnected to an oxidant gas discharge opening 24 c in the fuel cell 24and the air humidifier 25 is connected to the total heat exchanger 29.Thus, an air supply channel 30 for the oxidant gas of air is constitutedby connecting the air humidifier 25 to an oxidant gas supply opening 24d. Furthermore, an oxidant gas supply opening 24 d is disposed in anexternal manifold (not shown) mounted onto the upper part of the fuelcell stack for supplying the oxidant gas, and an oxidant gas dischargeopening 24 c is disposed in an external manifold (not shown) mountedonto the lower part of the fuel cell stack for discharging the oxidantgas. Thus, the fuel cell system is constituted such that the oxidant gasof air discharged from the oxidant gas channels in the respective platescomes altogether, and is discharged from the oxidant gas dischargeopening 24 c.

[0102] Moreover, a fuel reforming apparatus 31 is connected to the fuelhumidifier 26, so that the fuel reforming apparatus 31 converts a rawfuel such as town gas or the like to a reformed gas containing hydrogenas a main component. In the fuel cell system, the reformed gas ishumidified by the fuel humidifier 26 and then supplied to the fuel gassupply opening 24 e in the fuel cell 24. The humidification is carriedout by injecting the reformed gas into water stayed in the inside of thefuel humidifier 26. A wet fuel gas supplied to the fuel gas supplyopening 24 e In the fuel cell 24 is supplied into the connection holealigned in the stacking direction of the fuel cell stack by the fuel gassupply manifold, and then distributed fuel gas inlet headers in therespective plates, and further flows along the respective fuel gaschannels. The respective fuel gasses discharged from the fuel gaschannels (the fuel gas discharged without reaction) coming together inthe gas outlet section, and are discharged to the outside after passingthrough the connection hole in the stacking direction of the fuel cellstack. Generally, the fuel gas discharged to the outside withoutreaction is supplied from the fuel gas discharge opening 24 f to areforming burner in the fuel reforming apparatus and burned therein.

[0103] The air introduced as an oxidant gas from the outside exchangesthe heat with water by the total heat exchanger 29, and then it issupplied to the oxidant gas supply opening 24 d (in a more detailedexpression, the oxidant gas supply opening in the external manifold) ofthe fuel cell 24 via the air humidifier 25. Water stays in the inside ofthe air humidifier 25, and the air is humidified by injecting the airinto the water. A wet air supplied to the oxidant gas supply opening 24d of the fuel cell 24 is distributed into the gas inlet headers in therespective plates and flows along the respective oxidant gas channels.The airs discharged from the respective oxidant gas channels (the airsdischarged without reaction) comes together in the gas outlet header,and then discharged from the oxidant gas discharge opening 24 c in thefuel cell 24 (in a more detailed expression, the oxidant gas outlet inthe external manifold). The un-reacted air thus discharged is furtherdischarged to the outside via the total heat exchanger 29.

[0104] As described above, the fuel gas and the oxidant gas are suppliedto the fuel cell 24, so that an electrochemical reaction takes place viathe solid polymer electrolyte membrane in the cell (membrane electrodeassembly), thereby enabling a DC electric power to be generated. On theother hand, the water in the water circulating channel 28 is supplied tothe heat medium supply opening 24 b in the fuel cell 24, and flows theconnection hole in the stacking direction in the fuel cell stack, andthen distributed in the heat medium inlet headers of the respectiveanode cooling plates 22. The waters thus distributed flow along therespective heat medium channels, and the waters discharged from therespective heat medium channels come together in the heat medium outletheader. Thereafter, the water thus combined passes through theconnection hole aligned in the stacking direction of the cell stack, andfinally discharged from the heat medium discharge opening 24 a.

[0105] The anode cooling plate 22 is disposed such that the heat mediumchannels 22 b face the fuel gas channels 22 a on the other side, asdescribed above, thereby enabling the anode cooling plate 22 to becooled. Furthermore, the heat medium channels 22 b in the anode coolingplate 22 face the surface on which the oxidant gas channels of thecathode cooling plate 23 are formed, thereby enabling the cathodecooling plate 23 to be cooled. As a result, the fuel cell 22 is cooledduring the operation period in the electric power generation, andthereby enabling the fuel cell 22 to be maintained at a proper operationtemperature (about 80° C.).

[0106] On the other hand, the heat medium of water discharged from thefuel cell 24 is heated up at a temperature of 78° C. or so. When waterat such a high temperature is introduced into the air humidifier 25, thetemperature in the inside thereof can be increased. However, thetemperature of the water passed through the air humidifier 25 decreasesat 76° C. or so, and the water at such a medium temperature isintroduced into the fuel humidifier 26. The reformed gas at a hightemperature (100 to 150° C.) from the fuel reforming apparatus 31 isintroduced into the fuel humidifier 26, and injected into the water, asdescribe above, and therefore the water in the inside thereof ismaintained at 75 to 76° C. after losing the evaporation heat. The air issupplied to the air humidifier 25 after the dew point becomes 64° C. orso in the total heat exchanger 29.

[0107] The hot water passed through the fuel humidifier 26 is introducedinto the heat exchanger 27, in which the heat is exchanged between thehot water and the water supplied from a water reservoir (not shown), andthen the water is returned to the water reservoir after changed into ahot water. The temperature of the water passed through the heatexchanger 27 is decreased to 74° C. or so. The water at such a lowtemperature is supplied to the heat medium supply opening 24 b in thefuel cell 24. Accordingly, the heat in the cooling water as a heatmedium can be efficiently used by circulating the cooling waterdischarged from the fuel cell 24 via the water circulating channel 28.

[0108] In accordance with the present invention, the dew point of thereaction gas is set at the temperature of the heat medium or smaller inthe inlet area of the reaction gas, and the dew point of the reactiongas is set at the temperature of the heat medium or grater in the outletarea of the reaction gas.

[0109] Setting the temperature of the reaction gas less than temperatureof the heat medium causes the reaction gas to be heated up in the inletarea by the heat medium, thereby making it possible to prevent the watervapor in the wet reaction from dew condensation in the inlet area.Accordingly, the condensed water is not deposited onto the gas channelsin the inlet area of the reaction gas, so that the reaction gas startsto smoothly flow.

[0110] Setting the dew point of the reaction gas greater than thetemperature of the heat medium in the outlet area of the reaction gascauses the reaction gas to be cooled in the outlet area by the heatmedium, thereby the water vapor in the reaction gas to be occasionallycondensed. However, when the condensed water is adhered to the innerwall of the gas channels in the outlet area, a uniform pressure isapplied to the respective gas channels, thereby enabling the waterdroplets to be blown away. Accordingly, the condensed water can bedischarged in a short time to the gas outlet header. If the condensedwater is adhered to the inner wall of part of the gas channels, andthereby clogs the gas channels, as in the prior art, uniform gasdistribution does not occur in the respective gas channels, therebycausing an instable operation to take place in the power generation. Inaddition, the reaction gas is deflected to the other gas channel, hencemaking it difficult to blow away the water droplets. In accordance withthe present invention, as described above, the dew condensation iscompulsively carried out inside the respective gas channels in theoutlet area of the reaction gas, 80 that the pressure loss ishomogenized over the gas channels, thereby enabling a uniform gasdistribution to be attained.

[0111] In the above embodiments, it is described that the fuel gas flowsparallel to the oxidant gas from top to bottom in the direction ofgravity, whereas the heat medium flows in the direction anti-parallel tothe reaction gas. However, it is possible that the heat medium suppliesin the direction parallel to the reaction gas. In this case, it ispreferable that the cooling water discharged from the fuel cell 24 flowsin sequence from the heat recover heat exchanger 27, air humidifier 25,and the fuel humidifier 26, and then returned to the fuel cell 24.

[0112] While preferred embodiments have been shown and described,various modifications and substitutions may be made without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofexamples, and not by limitations.

What is claimed is:
 1. A fuel cell formed by stacking a plurality ofplates each having reaction gas channels or heat medium channels in afuel cell stack, wherein an inlet header for at least one side ofreaction gas channels is disposed so as to face the inlet header or theoutlet header for said heat medium channels.
 2. A fuel cell according toclaim 1, wherein the inlet header for said reaction gas channels ismaintained at the temperature of the dew point of gas or greater by theheat medium.
 3. A fuel cell formed by stacking a plurality of plateseach having reaction gas channels in a fuel cell stack, wherein theinlet header for one side of reaction gas channels is disposed such thatit faces the inlet header for the other side of reaction gas channels.4. A method for operating a fuel cell according to claim 3, wherein thedew point of one side of reaction gas is set at the temperature of theother side of reaction gas or smaller.
 5. A fuel cell formed by stackinga plurality of plates each having reaction gas channels or heat mediumchannels, wherein an inlet header for one side of reaction gas and aninlet header for the other side of reaction gas are disposed such thatthey face an inlet header or an outlet header for heat medium.
 6. A fuelcell according to claim 5, wherein the one side of reaction gas and theother side of reaction gas flow parallel to each other from top tobottom in the direction of gravity, and wherein both sides of reactiongas flow in direction parallel or anti-parallel to the heat medium.
 7. Afuel cell according to claim 5 or 6, wherein channels for the one sideof gas, the other side of gas and the heat medium are shaped straight inthe respective portions facing an anode or cathode electrode section. 8.A method for operating a fuel cell according to any one of claims 5 to7, wherein a following equation is established in the case when thereaction gases and the heat medium flow in the direction parallel toeach other, the dew point of at least one of the supplied reactiongases≧the temperature of the heat medium at the inlet, whereas ananother following equation is established in the case when the reactiongases and the heat medium flow in the direction anti-parallel to eachother, the dew point of at least one of the supplied reaction gases≦thetemperature of the heat medium at the outlet.
 9. A method for operatinga fuel cell according to claim 8, wherein a following equation isestablished in the case when the reaction gases and the heat medium flowin the direction parallel to each other, the dew point for at least oneside of discharged reaction gas≧the temperature of the heat medium atthe outlet, whereas an another equation is established in the case whenat least one of the reaction gases and the heat medium flow in thedirection anti-parallel to each other, the dew point for at least oneside of discharged reaction gas≧the temperature of the heat medium atthe inlet.
 10. A fuel cell according to claim 3, wherein the heat mediumis supplied so as to flow at an area facing the downstream area from thereaction gas inlet header, and wherein the heat medium heat-exchanged atan area facing the electrode section is supplied so as to flow at anarea facing the reaction gas inlet header.
 11. A method for operating afuel cell according to claim 10, wherein a following equation isestablished, the dew point for at least one side of reaction gas≧thetemperature of the heat medium at the inlet.
 12. A fuel cell and amethod for operating a fuel cell according to any one of claims 1 to 11,wherein a flow resistance generation section is disposed at the inlet ofat least one side of reaction gas.
 13. A fuel cell and a method foroperating a fuel cell according to claim 12, wherein the reaction gasinlet header includes the flow resistance generation section.
 14. A fuelcell system, wherein an oxidant humidifier and a fuel humidifier areconnected to the fuel cell according to any one of claims 1 to 13 andwherein the heat medium discharged from the fuel cell is heat-exchangedin these humidifiers.
 15. A fuel cell system, wherein an oxidanthumidifier and/or a fuel humidifier, and a total heat exchanger areconnected to a fuel cell, and the heat medium discharged from the fuelcell is heat-exchanged in these humidifiers, wherein the total heatexchange is carried out between at least one side of reaction gas in thereaction gas discharged from the fuel cell and at least one side ofreaction gas in the reaction gas before supplied to said humidifiers.16. A fuel cell system according to claim 15, wherein the fuel cell usedin the fuel cell system and the method for operating the same areidentical to the fuel cell and the fuel cell operating method defined byany one of claims 1 to 13, respectively.
 17. A fuel cell systemaccording to any one of claims 14 to 16, wherein the heat mediumdischarged from the fuel cell is first heat-exchanged in one humidifierin which either the oxidant gas or the fuel gas flows at a higher flowrate, and then heat-exchanged in the other humidifier.